Method for making a GaN electroluminescent semiconductor device utilizing epitaxial deposition

ABSTRACT

An electroluminescent semiconductor device comprising bodies of conductive and resistive crystalline gallium nitride (GaN) which are successively epitaxially deposited on a surface of a heat-treated sapphire substrate, and a body of insulative crystalline gallium nitride epitaxially deposited on the resistive body.

This is a division of application Ser. No. 213,599, filed Dec. 4, 1980,now U.S. Pat. No. 4,408,217.

BACKGROUND OF THE INVENTION

The present invention relates to electroluminescent semiconductordevices, and in particular to an electroluminescent semiconductor devicein which the active material is a body of gallium nitride. The inventionalso relates to a method for making such electroluminescentsemiconductor devices.

Electroluminescent semiconductor devices of gallium nitride are known inthe art. An example of such devices is shown and described in U.S. Pat.No. 3,683,240 granted to J. I. Pankove. The gallium nitrideelectroluminescent device described in this patent comprises a substrateof an electrically insulating material which is optically transparent,such as sapphire. On a surface of the substrate is a body of N typeconductive crystalline gallium nitride, which has a conductivity ofabout 100 mhos, and on the surface of this N type conductivity body is athin body of insulating crystalline gallium nitride. The conductive andinsulative bodies are epitaxially deposited on the sapphire substrate bythe vapor phase epitaxy technique. During the initial step of thedeposition process little or no acceptor impurity is included so thatthe initial portion of the deposited gallium nitride is conductive toform the conductive body. When this conductive body has attained adesired thickness, a sufficient amount of acceptor impurities isincluded so as to compensate all of the native, uncontrolled donors,such as nitrogen vacancies, which are inherently formed in the material,thereby forming the insulating gallium nitride body. One electricalcontact is provided on a surface of the insulating gallium nitride bodyand another contact is provided on the periphery of the conductivegallium nitride body. When a D.C. potential is applied between the twocontacts, high electric fields are generated in the insulating galliumnitride body which cause the release of electrons trapped in theacceptor centers and a subsequent avalanche multiplication of freeelectrons and holes. Light in the blue to green region is emitted by theinsulating gallium nitride body when these carriers are recombined andcan be seen through the substrate.

Electroluminescent semiconductor devices of gallium nitride are oftenemployed in conjunction with integrated circuits by sharing a commonD.C. source which is typically 12 volts or less. The operating voltageof gallium nitride electroluminescent devices is found to be largelydependent on the physical properties of the insulating gallium nitridebody, particularly, on the thickness of the insulating body. Theinsulating gallium nitride body having a thickness sufficiently small tomeet the low voltage requirement of integrated circuits will cause anelectrical breakdown in the body, which is detrimental to theelectroluminescent device. Therefore, the insulating gallium nitridebody is required to have a substantial thickness and therefore a highoperating voltage if reliable performance is to be assured.

In more detail, the conductive gallium nitride body is currentlyconsidered to have a heterogeneous structure due to latticeimperfections caused by different crystallographic properties of thematerial between different depths and by nonuniform growth rates atwhich the conductive gallium nitride is deposited. Therefore, theinsulative gallium nitride body that is deposited on such a conductivegallium nitride body tends to maintain the same lattice orientation asthe underlying body. Furthermore, such imperfections are particularlypronounced at the interface between the two bodies due to the differencein lattice coefficient between them. When a D.C. current is passedthrough the insulative body, the lattice imperfections in that bodybecome the centers of field concentration where avalanche multiplicationof free electrons and holes takes place to such an extent that theresistance value of the material in that field centers is reduced. Thisin turn enhances the field concentration and a large current will resultwhich breaks down the material. In applications where the thickness ofthe insulating gallium nitride body is held at a value less than 1micrometer, lattice imperfections thus become a factor which cannot beignored.

SUMMARY OF THE INVENTION

According to this invention, the electroluminescent semiconductor deviceincludes a resistive gallium nitride body which is sandwiched between anunderlying body of conductive gallium nitride and an overlying body ofinsulative gallium nitride, which are successively deposited by thevapor phase epitaxy technique on the surface of a sapphire substratewhich is heat-treated prior to the epitaxial deposition process. In theinitial step of deposition process, little or no donor impurities aredoped into the deposited gallium nitride. When a resistive galliumnitride body of the desired thickness is deposited, sufficient acceptorimpurities are doped into the gallium nitride to substantiallycompensate all of the native donors inherently formed in the material,thus making it insulative. The heat treatment of the sapphire substratecauses the crystallographic axes of the deposited gallium nitride toalign themselves. The high conductivity of the conductive galliumnitride body is due to the inherently formed native donors, or nitrogenvacancies, produced by the formation of numerous crystallized areas. Thesubsequently deposited gallium nitride maintains the initialcrystallographic orientation of the underlying body and during thesubsequent deposition process the areas that correspond to thecrystallized areas of the underlying body are fused together to form theoverlying, resistive body, so that the latter contains a lesser amountof native donors than the underlying, conductive body. A D.C. voltage isapplied between the insulating and conducting bodies in order for theinsulating body to generate therein an avalanche multiplication ofreleased electrons and holes. The resistive gallium nitride bodyprovides a current limiting action when the insulative body exhibits asharp drop in resistance in response to the generation of the avalancephenomenon. Electrical breakdown of the insulating body is successfullyprevented by the current limiting action of the resistive body, so thatthe thickness of the insulating body can be reduced to such an extentthat conforms to low operating voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described by way of example with referenceto the accompanying drawings, which:

FIG. 1 is a sectional view of an embodiment of the electroluminescentsemiconductor device of the invention;

FIG. 2 is a sectional view of a first modified form of the FIG. 1embodiment; and

FIG. 3 is a sectional view of a second modified form of the FIG. 1embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, an embodiment of the electroluminescentsemiconductor device of the invention is generally designated as 10. Theelectroluminescent semiconductor device 10 comprises a substrate 11 ofsapphire. On the substrate 11 are formed a body 12 of conductivecrystalline gallium nitride and a body 13 of resistive crystallinegallium nitride which are successively deposited on the substrate 11 bythe vapor phase epitaxy technique. On the resistive body 13 is a body 14of insulating crystalline gallium nitride. During the initial step ofprocess, little or no donor impurities are introduced into the bodies 12and 13 to make them conducting and resistive, respectively, as will beunderstood as description proceeds.

Prior to the deposition of conductive gallium nitride body 12, thesapphire substrate 11 was subjected to a surface activation whichinvolves heating a sapphire body at a temperature of between 850° C. and1100° C. in an environment containing a gallium compound. This galliumcompound is produced by source gallium contained in a boat which isheated at 850° C. The heated source gallium is reacted with a reactingagent which is selected from the group consisting of chlorine, iodineand bromine. The gallium compound GaX, where X is chlorine, iodine orbromine, in gaseous phase is formed and carried by a stream of purenitrogen gas to the sapphire substrate and deposited thereon. Thesurface activation is continued for a period of from 2 minutes to 2hours so that the activated surface is rendered amenable to epitaxialdeposition of gallium nitride.

During the deposition process of gallium nitride on the substrate 11,the deposited gallium nitride molecules initially coagulate intonumerous crystallized areas or islands which are subsequently grown andfused together to form the bodies 12 and 13. The body 12 corresponds toa region which contains imperfectly fused crystallized areas inabundance and as a result the native donors, such as nitrogen vacancieswhich are inherently formed in that region make the body 12 conductive.It is believed that the heat-treated surface of the sapphire substrate11 causes the crystallographic axes of the initially crystallized areasto align themselves, so that upon the subsequent fusion of galliumnitride the body 13 tends to contain a lesser amount of native donorsthan the underlying body 12, thus making the body 13 less conductivethan the body 12, but much more conductive than the overlying,insulative body 14.

When a resistive gallium nitride body 13 of the desired thickness,typically 50 micrometers, is deposited, a sufficient amount of acceptorimpurities is introduced into the gallium nitride body 14 to compensatesubstantially all of the native donors inherently formed in the galliumnitride to make the body 14 insulating. The deposition of the insulatingbody 14 is continued until a desired thickness, typically 1 micrometer,is attained.

The electroluminescent semiconductor device 10 further includes a metalcontact layer 15 of indium coated on the surface of the insulatinggallium nitride body 14. A metal contact layer 16, which is also ofindium, is coated on the periphery of the conductive gallium nitridebody 12 so that the conductive body 12 and the contact layer 15 serve asa contact to one side of the insulating body 14. Terminal wires 17 and18 are connected to the contact layers 15 and 16, respectively.

When the terminal wires 17 and 18 are connected to a source of directcurrent so as to apply a voltage of about 5 volts between the contacts15 and 16, avalanche multiplication of free electrons and holes occursand thus the resistance of the insulating body 14 sharply drops.However, the resistive body 13 serves to limit the amount of currentwhich would result from the resistance drop to a stabilized currentvalue. This assures recombination of electrons and holes in a stablizedmanner giving off light of either blue or green in color depending onthe concentration of the acceptor impurity in the insulating galliumnitride body 14.

Therefore, the provision of the resistive body 13 allows the insulatingbody 14 to have a sufficiently small thickness to be operable on lowvoltages while assuring a stable electroluminescent characteristic.

It was found that the preferred resistance value of the resistive body13 is between 100 ohms and 10 killoohms. The range for these resistancevalues in carrier concentration runs from 10¹⁵ cm⁻³ to 7×10¹⁷ cm⁻³ asconfirmed by ultraviolet spectrum analyses. It was also found that theresistive body 13 exhibits uniformity in the crystallographic structureand as a result the lattice imperfections of the insulative body 14 areconsiderably small compared to the insulating body of the prior artdevice.

FIGS. 2 and 3 are illustrations of modified forms of the embodiment ofFIG. 1. In FIGS. 2 and 3, parts which correspond to those of FIG. 1 aremarked with the same reference numerals as in FIG. 1, but preceded by anumeral representing the figure number. The parts having correspondingreference numerals in these figures have corresponding significance.

In FIG. 2, the surface of the sapphire substrate 21 is scratched in alimited area as at 21' prior to the epitaxial deposition of galliumnitride body 22. During the subsequent epitaxial deposition of galliumnitride bodies 22, 23 and 24, the portion of these bodies which isdeposited on the scratched area 21' tends to form a high conductivitycrystallized region 29. The high conductivity region 29 extends from theconductive body 22 upward to the surface of the insulative body 24. Ametal contact layer 26 is coated on the surface of the conductive region29. A terminal 28 is connected to the contact layer 26 so that thecontact layer 26 serves as a contact to one side of the insulating body14. The contact layer 26 may overlap the insulating gallium nitride body24 as long as the distance between the electrodes 25 and 26 is largecompared to the thickness of the insulating body 24. The highconductivity of the region 29 is considered to be attributed to the factthat the scratched surface 21' causes the initially deposited galliumnitride to form a large number of donors, or nitrogen vacancies, and thesubsequently deposited gallium nitride maintains the orientation of theinitially deposited gallium nitride crystals.

In FIG. 3, the electroluminescent semiconductor device 30 is formed witha recess 39 of V-shaped cross-section which extends from the surface ofthe insulating body 34 partially into the resistive body 33. An indiumcontact 36 is injected into the recess 39 and a terminal wire 38 isconnected to the contact 36. The contact 36 is spaced a distance fromthe contact 35 which is large compared to the thickness of theinsulating body 34 to permit the current to pass through the conductivebody 32.

What is claimed is:
 1. A method for making an electroluminescentsemiconductor device comprising the steps of:providing a heat treatmenton a body of sapphire in an environment containing a compound GaX, whereX is chlorine, iodine or bromine; successively epitaxially depositinggallium nitride on said sapphire body without doping to form a layer ofconductive region and an overlying layer of resistive region on saidconductive region; and epitaxially depositing gallium nitride on saidresistive region in an environment containing acceptor impurities toform an insulative layer.
 2. A method as claimed in claim 1, whereinsaid heat treatment is provided at a temperature higher than 850° C. forat least two minutes.
 3. A method as claimed in claim 1, furthercomprising the step of scratching a portion of a surface of saidsapphire body prior to said heat treatment to permit said successivelyepitaxially deposited gallium nitride to form a high conductivity regionextending from said scratched portion to a surface of said insulativelayer; and forming a pair of contacts electrically connected to spacedpoints on said surface of said insulative layer, one of said spacedpoints being in electrically contact with said high conductivity region.4. A method as claimed in claim 1, further comprising the step ofproviding a heat treatment on said sapphire body in an environmentcontaining a compound of GaX, where X is chlorine, iodine or brominebefore said gallium nitride is epitaxially deposited without doping.